This invention relates to a method and device for sealing a tear-off foil onto a packaging element.
Many consumer products are now sold in packagings which are closed with a tear-off foil. For example, foods such as yoghurt or similar are largely sold in pots sealed with a foil made of aluminium, plastic or a composite material. The seal is normally executed with the aid of a stamp tool which presses the foil against the opening edge under the influence of heat, so that a meltable seal coating of the foil enters into a firmly bonded connection with the edge.
Alternatively, it is possible to start by sealing only part of the packaging with a foil in the above-described way, an intermediate ring, for example, and to then connect this part with the rest of the packaging by means of crimping or some other prior art method. Within the meaning of this description, then, the term packaging element is understood to designate both complete packagings such as pots or similar, or parts thereof.
The process of heating the connecting point between packaging element and foil has thus far usually been performed with the help of heating elements disposed in the stamp tool. The heating temperature has to be hot enough to melt the foil sealing layer, whilst at the same, damage from overheating is to be avoided. This applies to the foil in particular, the top of which may be varnished or printed or similar. Furthermore, the stamp tool is located in the immediate vicinity of machine parts which should not be exposed to heat, e.g. punching tools to punch the foil out of a web of foil. Thermal expansion in said tools has to be compensated by enlarging the cutting play. This in turn limits the choice of foil material. Whilst cost reasons often make it desirable to use foils with a low percentage of aluminium, low thickness and high tear strength, the use of such materials is problematic in this context. The deployment of cooling devices in adjacent machine and tool parts to prevent excessive heating increases the complexity of the method, however, leading to greater costs.
Furthermore, the question of heat expansion has to be borne in mind in relation to the geometry of the packaging element to be sealed. If prior art sealing tools are used, the packaging elements must have a flat sealing surface in the form of, for example, an inwardly pointing edge, which does not facilitate extraction of the goods inside the packaging.
Another problem is that the process speed is dictated by the duration of the sealing step, during which the packaging element has to be brought up to the necessary sealing temperature by the heated stamp tool. This imposes limits on the scope for increasing the output of prior art sealing machines, and hence of reducing production costs.